ApplicationPoolServer.h

/*
 *  Phusion Passenger - http://www.modrails.com/
 *  Copyright (C) 2008  Phusion
 *
 *  Phusion Passenger is a trademark of Hongli Lai & Ninh Bui.
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; version 2 of the License.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License along
 *  with this program; if not, write to the Free Software Foundation, Inc.,
 *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
#ifndef _PASSENGER_APPLICATION_POOL_SERVER_H_
#define _PASSENGER_APPLICATION_POOL_SERVER_H_

#include <boost/shared_ptr.hpp>
#include <boost/thread/mutex.hpp>

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <cstdio>
#include <cstdlib>
#include <limits.h>
#include <errno.h>
#include <unistd.h>
#include <signal.h>

#include "MessageChannel.h"
#include "ApplicationPool.h"
#include "Application.h"
#include "Exceptions.h"
#include "Logging.h"
#include "System.h"

namespace Passenger {

using namespace std;
using namespace boost;


/**
 * Multi-process usage support for ApplicationPool.
 *
 * ApplicationPoolServer implements a client/server architecture for ApplicationPool.
 * This allows one to use ApplicationPool in a multi-process environment (unlike
 * StandardApplicationPool). The cache/pool data is stored in the server. Different
 * processes can then access the pool through the server.
 *
 * ApplicationPoolServer itself does not inherit ApplicationPool. Instead, it returns
 * an ApplicationPool object via the connect() call. For example:
 * @code
 *   // Create an ApplicationPoolServer.
 *   ApplicationPoolServer server(...);
 *   
 *   // Now fork a child process, like Apache's prefork MPM eventually will.
 *   pid_t pid = fork();
 *   if (pid == 0) {
 *       // Child process
 *       
 *       // Connect to the server. After connection, we have an ApplicationPool
 *       // object!
 *       ApplicationPoolPtr pool(server.connect());
 *
 *       // We don't need to connect to the server anymore, so we detach from it.
 *       // This frees up some resources, such as file descriptors.
 *       server.detach();
 *
 *       ApplicationPool::SessionPtr session(pool->get("/home/webapps/foo"));
 *       do_something_with(session);
 *
 *       _exit(0);
 *   } else {
 *       // Parent process
 *       waitpid(pid, NULL, 0);
 *   }
 * @endcode
 *
 * <h2>Implementation notes</h2>
 *
 * <h3>Separate server executable</h3>
 * The actual server is implemented in ApplicationPoolServerExecutable.cpp, this class is
 * just a convenience class for starting/stopping the server executable and connecting
 * to it.
 *
 * In the past, the server logic itself was implemented in this class. This implies that
 * the ApplicationPool server ran inside the Apache process. This presented us with several
 * problems:
 * - Because of the usage of threads in the ApplicationPool server, the Apache VM size would
 *   go way up. This gave people the (wrong) impression that Passenger uses a lot of memory,
 *   or that it leaks memory.
 * - Although it's not entirely confirmed, we suspect that it caused heap fragmentation as
 *   well. Apache allocates lots and lots of small objects on the heap, and ApplicationPool
 *   server isn't exactly helping. This too gave people the (wrong) impression that
 *   Passenger leaks memory.
 * - It would unnecessarily bloat the VM size of Apache worker processes.
 * - We had to resort to all kinds of tricks to make sure that fork()ing a process doesn't
 *   result in file descriptor leaks.
 * - Despite everything, there was still a small chance that file descriptor leaks would
 *   occur, and this could not be fixed. The reason for this is that the Apache control
 *   process may call fork() right after the ApplicationPool server has established a new
 *   connection with a client.
 *
 * Because of these problems, it was decided to split the ApplicationPool server to a
 * separate executable. This comes with no performance hit.
 *
 * <h3>Anonymous server socket</h3>
 * Notice that ApplicationPoolServer does do not use TCP sockets at all, or even named Unix
 * sockets, despite being a server that can handle multiple clients! So ApplicationPoolServer
 * will expose no open ports or temporary Unix socket files. Only child processes are able
 * to use the ApplicationPoolServer.
 *
 * This is implemented through anonymous Unix sockets (<tt>socketpair()</tt>) and file descriptor
 * passing. It allows one to emulate <tt>accept()</tt>. ApplicationPoolServer is connected to
 * the server executable through a Unix socket pair. connect() sends a connect request to the
 * server through that socket. The server will then create a new socket pair, and pass one of
 * them back. This new socket pair represents the newly established connection.
 *
 * @ingroup Support
 */
class ApplicationPoolServer {
private:
        /**
         * Contains data shared between RemoteSession and Client.
         * Since RemoteSession and Client have different life times, i.e. one may be
         * destroyed before the other, they both use a smart pointer that points to
         * a SharedData. This way, the SharedData object is only destroyed when
         * both the RemoteSession and the Client object has been destroyed.
         */
        struct SharedData {
                /**
                 * The socket connection to the ApplicationPool server, as was
                 * established by ApplicationPoolServer::connect().
                 */
                int server;
                
                mutex lock;
                
                ~SharedData() {
                        int ret;
                        do {
                                ret = close(server);
                        } while (ret == -1 && errno == EINTR);
                }
        };
        
        typedef shared_ptr<SharedData> SharedDataPtr;
        
        /**
         * An Application::Session which works together with ApplicationPoolServer.
         */
        class RemoteSession: public Application::Session {
        private:
                SharedDataPtr data;
                int id;
                int fd;
                pid_t pid;
        public:
                RemoteSession(SharedDataPtr data, pid_t pid, int id, int fd) {
                        this->data = data;
                        this->pid = pid;
                        this->id = id;
                        this->fd = fd;
                }
                
                virtual ~RemoteSession() {
                        closeStream();
                        mutex::scoped_lock(data->lock);
                        MessageChannel(data->server).write("close", toString(id).c_str(), NULL);
                }
                
                virtual int getStream() const {
                        return fd;
                }
                
                virtual void shutdownReader() {
                        if (fd != -1) {
                                int ret = InterruptableCalls::shutdown(fd, SHUT_RD);
                                if (ret == -1) {
                                        throw SystemException("Cannot shutdown the writer stream",
                                                errno);
                                }
                        }
                }
                
                virtual void shutdownWriter() {
                        if (fd != -1) {
                                int ret = InterruptableCalls::shutdown(fd, SHUT_WR);
                                if (ret == -1) {
                                        throw SystemException("Cannot shutdown the writer stream",
                                                errno);
                                }
                        }
                }
                
                virtual void closeStream() {
                        if (fd != -1) {
                                int ret = InterruptableCalls::close(fd);
                                if (ret == -1) {
                                        throw SystemException("Cannot close the session stream",
                                                errno);
                                }
                                fd = -1;
                        }
                }
                
                virtual void discardStream() {
                        fd = -1;
                }
                
                virtual pid_t getPid() const {
                        return pid;
                }
        };
        
        /**
         * An ApplicationPool implementation that works together with ApplicationPoolServer.
         * It doesn't do much by itself, its job is mostly to forward queries/commands to
         * the server and returning the result. Most of the logic is in the server executable.
         */
        class Client: public ApplicationPool {
        private:
                // The smart pointer only serves to keep the shared data alive.
                // We access the shared data via a normal pointer, for performance.
                SharedDataPtr dataSmartPointer;
                SharedData *data;
                
        public:
                /**
                 * Create a new Client.
                 *
                 * @param sock The newly established socket connection with the ApplicationPoolServer.
                 */
                Client(int sock) {
                        dataSmartPointer = ptr(new SharedData());
                        data = dataSmartPointer.get();
                        data->server = sock;
                }
                
                virtual void clear() {
                        MessageChannel channel(data->server);
                        mutex::scoped_lock l(data->lock);
                        channel.write("clear", NULL);
                }
                
                virtual void setMaxIdleTime(unsigned int seconds) {
                        MessageChannel channel(data->server);
                        mutex::scoped_lock l(data->lock);
                        channel.write("setMaxIdleTime", toString(seconds).c_str(), NULL);
                }
                
                virtual void setMax(unsigned int max) {
                        MessageChannel channel(data->server);
                        mutex::scoped_lock l(data->lock);
                        channel.write("setMax", toString(max).c_str(), NULL);
                }
                
                virtual unsigned int getActive() const {
                        MessageChannel channel(data->server);
                        mutex::scoped_lock l(data->lock);
                        vector<string> args;
                        
                        channel.write("getActive", NULL);
                        channel.read(args);
                        return atoi(args[0].c_str());
                }
                
                virtual unsigned int getCount() const {
                        MessageChannel channel(data->server);
                        mutex::scoped_lock l(data->lock);
                        vector<string> args;
                        
                        channel.write("getCount", NULL);
                        channel.read(args);
                        return atoi(args[0].c_str());
                }
                
                virtual void setMaxPerApp(unsigned int max) {
                        MessageChannel channel(data->server);
                        mutex::scoped_lock l(data->lock);
                        channel.write("setMaxPerApp", toString(max).c_str(), NULL);
                }
                
                virtual pid_t getSpawnServerPid() const {
                        this_thread::disable_syscall_interruption dsi;
                        MessageChannel channel(data->server);
                        mutex::scoped_lock l(data->lock);
                        vector<string> args;
                        
                        channel.write("getSpawnServerPid", NULL);
                        channel.read(args);
                        return atoi(args[0].c_str());
                }
                
                virtual Application::SessionPtr get(
                        const string &appRoot,
                        bool lowerPrivilege = true,
                        const string &lowestUser = "nobody",
                        const string &environment = "production",
                        const string &spawnMethod = "smart",
                        const string &appType = "rails"
                ) {
                        this_thread::disable_syscall_interruption dsi;
                        MessageChannel channel(data->server);
                        mutex::scoped_lock l(data->lock);
                        vector<string> args;
                        int stream;
                        bool result;
                        
                        try {
                                channel.write("get", appRoot.c_str(),
                                        (lowerPrivilege) ? "true" : "false",
                                        lowestUser.c_str(),
                                        environment.c_str(),
                                        spawnMethod.c_str(),
                                        appType.c_str(),
                                        NULL);
                        } catch (const SystemException &) {
                                throw IOException("The ApplicationPool server exited unexpectedly.");
                        }
                        try {
                                result = channel.read(args);
                        } catch (const SystemException &e) {
                                throw SystemException("Could not read a message from "
                                        "the ApplicationPool server", e.code());
                        }
                        if (!result) {
                                throw IOException("The ApplicationPool server unexpectedly "
                                        "closed the connection.");
                        }
                        if (args[0] == "ok") {
                                stream = channel.readFileDescriptor();
                                return ptr(new RemoteSession(dataSmartPointer,
                                        atoi(args[1]), atoi(args[2]), stream));
                        } else if (args[0] == "SpawnException") {
                                if (args[2] == "true") {
                                        string errorPage;
                                        
                                        if (!channel.readScalar(errorPage)) {
                                                throw IOException("The ApplicationPool server "
                                                        "unexpectedly closed the connection.");
                                        }
                                        throw SpawnException(args[1], errorPage);
                                } else {
                                        throw SpawnException(args[1]);
                                }
                        } else if (args[0] == "BusyException") {
                                throw BusyException(args[1]);
                        } else if (args[0] == "IOException") {
                                throw IOException(args[1]);
                        } else {
                                throw IOException("The ApplicationPool server returned "
                                        "an unknown message: " + toString(args));
                        }
                }
        };
        
        
        static const int SERVER_SOCKET_FD = 3;
        
        string m_serverExecutable;
        string m_spawnServerCommand;
        string m_logFile;
        string m_rubyCommand;
        string m_user;
        string statusReportFIFO;
        
        /**
         * The PID of the ApplicationPool server process. If no server process
         * is running, then <tt>serverPid == 0</tt>.
         *
         * @invariant
         *    if serverPid == 0:
         *       serverSocket == -1
         */
        pid_t serverPid;
        
        /**
         * The connection to the ApplicationPool server process. If no server
         * process is running, then <tt>serverSocket == -1</tt>.
         *
         * @invariant
         *    if serverPid == 0:
         *       serverSocket == -1
         */
        int serverSocket;
        
        /**
         * Shutdown the currently running ApplicationPool server process.
         *
         * @pre System call interruption is disabled.
         * @pre serverSocket != -1 && serverPid != 0
         * @post serverSocket == -1 && serverPid == 0
         */
        void shutdownServer() {
                this_thread::disable_syscall_interruption dsi;
                int ret;
                time_t begin;
                bool done = false;
                
                InterruptableCalls::close(serverSocket);
                if (!statusReportFIFO.empty()) {
                        do {
                                ret = unlink(statusReportFIFO.c_str());
                        } while (ret == -1 && errno == EINTR);
                }
                
                P_TRACE(2, "Waiting for existing ApplicationPoolServerExecutable (PID " <<
                        serverPid << ") to exit...");
                begin = InterruptableCalls::time(NULL);
                while (!done && InterruptableCalls::time(NULL) < begin + 5) {
                        /*
                         * Some Apache modules fork(), but don't close file descriptors.
                         * mod_wsgi is one such example. Because of that, closing serverSocket
                         * won't always cause the ApplicationPool server to exit. So we send it a
                         * signal.
                         */
                        InterruptableCalls::kill(serverPid, SIGINT);
                        
                        ret = InterruptableCalls::waitpid(serverPid, NULL, WNOHANG);
                        done = ret > 0 || ret == -1;
                        if (!done) {
                                InterruptableCalls::usleep(100000);
                        }
                }
                if (done) {
                        P_TRACE(2, "ApplicationPoolServerExecutable exited.");
                } else {
                        P_DEBUG("ApplicationPoolServerExecutable not exited in time. Killing it...");
                        InterruptableCalls::kill(serverPid, SIGTERM);
                        InterruptableCalls::waitpid(serverPid, NULL, 0);
                }
                
                serverSocket = -1;
                serverPid = 0;
        }
        
        /**
         * Start an ApplicationPool server process. If there's already one running,
         * then the currently running one will be shutdown.
         *
         * @pre System call interruption is disabled.
         * @post serverSocket != -1 && serverPid != 0
         * @throw SystemException Something went wrong.
         */
        void restartServer() {
                int fds[2];
                pid_t pid;
                
                if (serverPid != 0) {
                        shutdownServer();
                }
                
                if (InterruptableCalls::socketpair(AF_UNIX, SOCK_STREAM, 0, fds) == -1) {
                        throw SystemException("Cannot create a Unix socket pair", errno);
                }
                
                createStatusReportFIFO();
                
                pid = InterruptableCalls::fork();
                if (pid == 0) { // Child process.
                        dup2(fds[0], SERVER_SOCKET_FD);
                        
                        // Close all unnecessary file descriptors
                        for (long i = sysconf(_SC_OPEN_MAX) - 1; i > SERVER_SOCKET_FD; i--) {
                                close(i);
                        }
                        
                        execlp(
                                #if 0
                                        "valgrind",
                                        "valgrind",
                                #else
                                        m_serverExecutable.c_str(),
                                #endif
                                m_serverExecutable.c_str(),
                                toString(Passenger::getLogLevel()).c_str(),
                                m_spawnServerCommand.c_str(),
                                m_logFile.c_str(),
                                m_rubyCommand.c_str(),
                                m_user.c_str(),
                                statusReportFIFO.c_str(),
                                NULL);
                        int e = errno;
                        fprintf(stderr, "*** Passenger ERROR: Cannot execute %s: %s (%d)\n",
                                m_serverExecutable.c_str(), strerror(e), e);
                        fflush(stderr);
                        _exit(1);
                } else if (pid == -1) { // Error.
                        InterruptableCalls::close(fds[0]);
                        InterruptableCalls::close(fds[1]);
                        throw SystemException("Cannot create a new process", errno);
                } else { // Parent process.
                        InterruptableCalls::close(fds[0]);
                        serverSocket = fds[1];
                        serverPid = pid;
                }
        }
        
        void createStatusReportFIFO() {
                char filename[PATH_MAX];
                int ret;
                
                snprintf(filename, sizeof(filename), "/tmp/passenger_status.%d.fifo",
                        getpid());
                filename[PATH_MAX - 1] = '\0';
                do {
                        ret = mkfifo(filename, S_IRUSR | S_IWUSR);
                } while (ret == -1 && errno == EINTR);
                if (ret == -1 && errno != EEXIST) {
                        int e = errno;
                        P_WARN("*** WARNING: Could not create FIFO '" << filename <<
                                "': " << strerror(e) << " (" << e << ")" << endl <<
                                "Disabling Passenger ApplicationPool status reporting.");
                        statusReportFIFO = "";
                } else {
                        statusReportFIFO = filename;
                }
        }

public:
        /**
         * Create a new ApplicationPoolServer object.
         *
         * @param serverExecutable The filename of the ApplicationPool server
         *            executable to use.
         * @param spawnServerCommand The filename of the spawn server to use.
         * @param logFile Specify a log file that the spawn server should use.
         *            Messages on its standard output and standard error channels
         *            will be written to this log file. If an empty string is
         *            specified, no log file will be used, and the spawn server
         *            will use the same standard output/error channels as the
         *            current process.
         * @param rubyCommand The Ruby interpreter's command.
         * @param user The user that the spawn manager should run as. This
         *             parameter only has effect if the current process is
         *             running as root. If the empty string is given, or if
         *             the <tt>user</tt> is not a valid username, then
         *             the spawn manager will be run as the current user.
         * @throws SystemException An error occured while trying to setup the spawn server
         *            or the server socket.
         * @throws IOException The specified log file could not be opened.
         */
        ApplicationPoolServer(const string &serverExecutable,
                     const string &spawnServerCommand,
                     const string &logFile = "",
                     const string &rubyCommand = "ruby",
                     const string &user = "")
        : m_serverExecutable(serverExecutable),
          m_spawnServerCommand(spawnServerCommand),
          m_logFile(logFile),
          m_rubyCommand(rubyCommand),
          m_user(user) {
                serverSocket = -1;
                serverPid = 0;
                this_thread::disable_syscall_interruption dsi;
                restartServer();
        }
        
        ~ApplicationPoolServer() {
                if (serverSocket != -1) {
                        this_thread::disable_syscall_interruption dsi;
                        shutdownServer();
                }
        }
        
        /**
         * Connects to the server and returns a usable ApplicationPool object.
         * All cache/pool data of this ApplicationPool is actually stored on
         * the server and shared with other clients, but that is totally
         * transparent to the user of the ApplicationPool object.
         *
         * @note
         *   All methods of the returned ApplicationPool object may throw
         *   SystemException, IOException or boost::thread_interrupted.
         *
         * @warning
         * One may only use the returned ApplicationPool object for handling
         * one session at a time. For example, don't do stuff like this:
         * @code
         *   ApplicationPoolPtr pool = server.connect();
         *   Application::SessionPtr session1 = pool->get(...);
         *   Application::SessionPtr session2 = pool->get(...);
         * @endcode
         * Otherwise, a deadlock can occur under certain circumstances.
         * @warning
         * Instead, one should call connect() multiple times:
         * @code
         *   ApplicationPoolPtr pool1 = server.connect();
         *   Application::SessionPtr session1 = pool1->get(...);
         *   
         *   ApplicationPoolPtr pool2 = server.connect();
         *   Application::SessionPtr session2 = pool2->get(...);
         * @endcode
         *
         * @throws SystemException Something went wrong.
         * @throws IOException Something went wrong.
         */
        ApplicationPoolPtr connect() {
                try {
                        this_thread::disable_syscall_interruption dsi;
                        MessageChannel channel(serverSocket);
                        int clientConnection;
                        
                        // Write some random data to wake up the server.
                        channel.writeRaw("x", 1);
                        
                        clientConnection = channel.readFileDescriptor();
                        return ptr(new Client(clientConnection));
                } catch (const SystemException &e) {
                        throw SystemException("Could not connect to the ApplicationPool server", e.code());
                } catch (const IOException &e) {
                        string message("Could not connect to the ApplicationPool server: ");
                        message.append(e.what());
                        throw IOException(message);
                }
        }
        
        /**
         * Detach the server, thereby telling it that we don't want to connect
         * to it anymore. This frees up some resources in the current process,
         * such as file descriptors.
         *
         * This method is particularily useful to Apache worker processes that
         * have just established a connection with the ApplicationPool server.
         * Any sessions that are opened prior to calling detach(), will keep
         * working even after a detach().
         *
         * This method may only be called once. The ApplicationPoolServer object
         * will become unusable once detach() has been called, so call connect()
         * before calling detach().
         */
        void detach() {
                int ret;
                do {
                        ret = close(serverSocket);
                } while (ret == -1 && errno == EINTR);
                serverSocket = -1;
        }
};

typedef shared_ptr<ApplicationPoolServer> ApplicationPoolServerPtr;

} // namespace Passenger

#endif /* _PASSENGER_APPLICATION_POOL_SERVER_H_ */

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